CoCo, it may need a "tune up" New plugs,plug wires,pvc valve,air cleaner,oil changeand filter. Do you have a 'Check Engine'light on? This could diagnose a need for an EGR module / sensor or an oxygen sensor(s) . Also , a problem with a having a car tested for emmision is it doensn't have time to warm up enough. Ya might try taking it for a 30-50 mi ride up and down the interstate or 2 lane to get it warmed up enough o pass cause a cold engine will emit more emmisions than a 'warm / hot engine will
they should have told you what it might be is so many things if he was any kind of a smog guy he would have... take it somewhere else and have it check tell them what you are wanting to do could be it needs a tune up but could be any of a few parts most of the time its a tune up
you,ll have to get the computer scanned on it to see if it has any trouble codes in it,this can cause it to fail,that means its running too rich,it could be an 0-2 sensor,causing this,but have it scanned to be sure,then you,ll know,good luck with it.
Check out the OXYGEN (O2) SENSOR OPERATION AND TESTING over @ the (first) link below.
Plus, there is a way to read the codes listed in these link w/o havin a scanner. If, you want to go that route. Or just run it down to the local AutoZone or Advanced Auto and they will read the code for you.